1. Field of the Invention
The present invention relates to a reversing sheet transport apparatus capable of reversing the side of a sheet of paper and an image forming apparatus including such a reversing sheet transport apparatus.
2. Description of the Related Art
In the above type of reversing sheet transport apparatus, transport of a sheet of paper by registration rollers is started after an edge of the sheet is hit against the registration rollers that are at a temporary stop and aligned. In the case of reversing the side of the sheet, reverse rollers are once stopped and then rotated in the reverse direction midway through the transport of the sheet by the reverse rollers, to transport the sheet back to the registration rollers through a reverse transport path by the reverse rollers. An edge of the sheet is hit again against the registration rollers that are at a temporary stop, and then the sheet is transported again by the registration rollers.
For example, in JP 2002-60095A, switch-back rollers (corresponding to the reverse rollers) are rotated in the reverse direction midway through the transport of an original by the switch-back rollers, to transport the original to the registration rollers through a return path (corresponding to the reverse transport path) by the switch-back rollers. An edge of the original is hit again against the registration rollers that are at a temporary stop, and then the original is transported again by the registration rollers, thereby revering the side of the original.
Such a reversing sheet transport apparatus is provided with an electromagnetic clutch that conveys or shuts off the rotation of a motor to the registration rollers, and the registration rollers are temporarily stopped by disengaging the electromagnetic clutch.
In some reversing sheet transport apparatuses, both the registration rollers and the reverse rollers are rotated by a single motor. In this case, in order to reverse the side of a sheet of paper, the rotation of the motor is reversed to rotate the reverse rollers in the reverse direction, to allow the reverse rollers to send the sheet back to the registration rollers through the reverse transport path. Thereafter, the motor is rotated normally, to allow the registration rollers to transport the sheet again. However, at the time of start of re-transport of the sheet by the registration rollers, the sheet still partly lies on the reverse rollers. Therefore, when the motor is rotated normally to allow both the reverse rollers and the registration rollers to rotate in the same direction, the sheet is pulled back by the reverse rollers with an edge of the sheet separating from the registration rollers. To address this problem, another electromagnetic clutch is provided to permit or block conveyance of the rotation of the motor to the reverse rollers, and the electromagnetic clutch for the reverse rollers is disengaged (when the reverse rotation of the motor is stopped, for example) to render the reverse rollers freely rotatable temporarily.
However, when a sheet of paper is thick and rigid, the reverse rollers tend to drag the sheet when the electromagnetic clutch for the reverse rollers is disengaged. Therefore, the sheet is pushed back toward the reverse rollers due to the elastic force of the sheet that has hit against the registration rollers and bent. This results in separation of the edge of the sheet from the registration rollers in a short period of time before the registration rollers restart transport of the sheet, causing jamming.
In JP 2002-60095A mentioned above, in which a motor for switch-back rollers is provided in addition to the motor for the registration rollers, it is unnecessary to provide an electromagnetic clutch for the switch-back rollers separately. However, the provision of two exclusive motors increases the scale of the apparatus and complicates the apparatus, increasing the cost of the apparatus.